This invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of manufacturing a silicon-on insulator(SOI) substrate having an isolation film therein.
Typically, a SOI wafer has a structure where an insulating layer is sandwiched between a substrate for supporting the SOI wafer and a substrate which a device is formed therein. Devices using the SOI wafer has advantages of complete isolation, parasitic capacitance reduction and high speed. There are typically a separation by implanted oxygen (SIMOX) method and a bonding method as a fabrication method of the SOI wafer. The SIMOX method is to fabricate a SOI wafer by implanting oxygens into a silicon substrate. The method of fabricating a SOI wafer by using a SIMOX method has a disadvantage that it is difficult to control a thickness of a silicon layer which a device is formed therein, because of forming a buried insulating layer by ion implantation. Recently, the bonding method is generally used for fabricating a SOI wafer.
FIG. 1 is a sectional view of a SOI wafer having an isolation film. A first silicon substrate 11 is prepared. Trenches are formed in the first silicon substrate 11 and a silicon oxide film 12 which is used for a polishing stopper in the following chemical mechanical polishing (CMP), is filled in the trenches. The silicon oxide film 12 is polished by a CMP process until an upper surface of the first silicon substrate is exposed, thereby forming an isolation film of the silicon oxide filled in the trenches. A buried insulating layer 14 of an oxide film is formed on the first silicon substrate 11 which the isolation film is formed therein. A second silicon substrate 13 is prepared and the first silicon substrate 11 and the second silicon substrate 13 are bonded to contact the oxide film 14 with the upper surface of the second silicon substrate 13. A heat treatment is carried out to intensify the bonding force between the first and the second substrates 11 and 13. The first silicon substrate 11 is back-side polished until the isolation film 12 of the oxide film filled in the trenches is exposed, thereby fabricating a SOI wafer 100. The remaining first silicon substrate 11 becomes a silicon active layer and the thickness of the remaining first silicon substrate 11 becomes a desired thickness of an active layer of a device formed on the SOI wafer 100.
In the prior SOI wafer fabrication method, although the silicon oxide film 12 for the isolation film buried in trenches is used for a polishing stopper in CMP process in order to obtain an uniform and thin silicon active layer, the dishing phenomenon as shown "A" in FIG. 1 that the central portion of the silicon active layer 11 is recessed rather than peripheral portion of the silicon active layer, is caused. In order to solve the problem, another method that stabilize the CMP process by controlling a concentration of slurry and a degree of acidity pH of slurry used in the CMP process, had been proposed. When the method is applied to the substrate which isolation regions having the same widths are formed therein, the dishing phenomenon is not caused. However, when the method is applied to the substrate which isolation regions having the different widths each other are formed therein, the dishing phenomenon is still exist.